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Study Objective.To assess the pharmacokinetics of levofloxacin during continuous venovenous hemodiafiltration (CVVHDF) and continuous venovenous hemofiltration (CVVH).

Thiopurine methyltransferase (TPMT) catalyzes the S-methylation of thiopurine drugs. TPMT activity is regulated by a common genetic polymorphism that is associated with large individual variations in thiopurine toxicity and efficacy. We previously cloned the functional gene for human TPMT and reported a common variant allele for low enzyme activity, TPMT*3A, that contains point mutations at cDNA nucleotides 460 and 719. In the present study, we set out to determine the number, types, and frequencies of TPMT variant alleles associated with low enzyme activity in clinical laboratory samples in the United States and to compare those results with data obtained from two different ethnic groups. We identified a total of six different variant alleles for low TPMT activity in the 283 clinical laboratory samples studied. The most common variant was *3A; the second most frequent variant allele, *3C, contained only the nucleotide 719 polymorphism; and four other variant alleles were detected. TPMT*3A also appeared to be the most common variant allele in a Norwegian white population sample, but it was not found in a population sample of Korean children. However, *3C was present in samples from the Korean children, as was a novel allele, *6. Characterization of variant alleles for low TPMT enzyme activity will help make it possible to assess the potential clinical utility of deoxyribonucleic acid-based diagnostic tests for determining TPMT genotype.

Anaesthetized rabbits were infused with methotrexate (MTX; 30 µg × kg−1× min−1for 4h. Constant plasma concentrations of MTX and its main metabolite 7-hydroxymethotrexate (7-OH-MTX) were achieved 40–60 min after the start of the infusion. In all, 50% of the infused MTX was eliminated by the kidney; another 15%–30% was hydroxylated and excreted as 7-OH-MTX in the urine. A concomitant infusion of penicillin G (3.96 mg × kg−1× min−1) decreased the renal clearance of MTX and 7-OH-MTX, probably by competitive antagonism at the common tubular secretion site. In contrast, the four cephalosporins ceftriaxone, ceftazidime, ceftizoxime and cefoperazone all increased the renal clearance of MTX and 7-OH-MTX. At similar plasma concentrations, ceftriaxone and ceftazidime were almost equipotent, ceftizoxime was less effective and cefoperazone seemed to have a biphasic effect, depressing the clearance of MTX and 7-OH-MTX at higher drug concentrations. The effects are best explained by an inhibition of the tubular reabsorption of the cytostatic and its metabolite. The results suggest that cephalosporins are a better choice than penicillin for antibiotic treatment during MTX therapy.

Summary In pentobarbital anesthetized rats that received 4 mg/kg i.v. methotrexate (MTX) or 7-hydroxymethotrexate (7-OH-MTX), the pharmacokinetics of the two drugs were similar. Plasma concentrations of both drugs declined biexponentially, with terminal half-lives of 90.6 min for MTX and 97.2 min for 7-OH-MTX. The total clearance values were 9.2 and 9.6 mlxkg-1xmin-1, respectively. With MTX, 48.2% of the dose was excreted in the urine within 200 min and another 31.6% was recovered from the bile; 5.8% was metabolized to 7-OH-MTX and appeared in the bile. Plasma concentrations of the metabolite 7-OH-MTX after MTX administration were below the detection limit. Injected 7-OH-MTX was predominantly excreted into the bile (72.8% of the dose); only 11.2% could be recovered from the urine. Differences between the physicochemical properties of MTX and 7-OH-MTX or different affinities for active transport systems may account for the unequal importance of these two excretion pathways for the two compounds.

In rabbits the IV kinetics of MTX (1.33, 4 and 12 mg/kg) could be described by a linear three-compartment model with a terminal half-life between 2.4 and 3.6 h. During 8 h 50% of the dose was excreted into urine in unchanged form and 15% as the metabolite 7-OH-MTX. These fractions remained constant with increasing dose. In continuous infusion experiments (9–900 µg/kg x min MTX IV) a decrease of the renal MTX clearance with increasing plasma concentration was observed. This effect was nearly compensated by an increase of the extrarenal MTX clearance. After short-term infusion of 7-OH-MTX (4 mg/kg) a biexponential decline of 7-OH-MTX plasma concentrations was observed with a terminal half-life of 0.45 h. About 80% of the dose was regained from urine during 5 h. From the combined pharmacokinetic data a linear model was constructed for the calculation of 7-OH-MTX plasma concentrations after short-term MTX infusion. For the first 4 h after MTX application the predicted values were in good accordance with the 7-OH-MTX concentrations actually measured.

In rabbits methotrexate (MTX) plasma concentrations showed a triexponential decline after short-term infusion (12 mg/kg in 10 min). Probenecid (50 mg/kg PO 1 h before MTX) and piperacillin (100 mg/kg SC 10 min before MTX plus 50 mg/kg SC 4 h later) increased the plasma concentrations of MTX and its metabolite 7-hydroxy-methotrexate (7-OH-MTX) 40 min to 6 h after the end of MTX infusion. The total body clearance of MTX was reduced, while the elimination half-life and the drug distribution to the peripheral body compartments were unchanged. Doxycycline (5 mg/kg PO 30 min before MTX) had no influence on the pharmacokinetics of MTX. Tobramycin was ineffective when given either only on the day of MTX infusion (2 mg/kg SC, 5 min before MTX and 2 mg/kg SC 4 h later) or as a 5-day pretreatment (2X2 mg/kg SC daily). The influence of probenecid and piperacillin can be explained by reduction of the renal elimination of MTX via the tubular transport mechanism for organic acids. This interaction occurred with therapeutic drug concentrations and thus may be clinically relevant.

In anesthetized rabbits, continuous infusion of methotrexate (MTX; 30 µg kg-1 min-1) established steadystate plasma concentrations for MTX and the metabolite 7-hydroxymethotrexate (7-OH-MTX) within 40 min. Fifty percent of the infused dose was eliminated unchanged by the kidneys and the renal MTX clearance was slightly higher than the inulin clearance. Another 15%–30% was metabolized and excreted as 7-OH-MTX in the urine. Infusions of 7-OH-MTX, furosemide or benzarone had no influence on the clearance of MTX or 7-OH-MTX. Infusions of probenecid or piperacillin decreased the renal clearance of MTX and 7-OH-MTX mainly by reducing the tubular secretion of both compounds. In contrast, infusions of the antibiotics ceftriaxone and sulfamethoxazole increased the renal elimination of MTX and 7-OH-MTX. An increase was also observed during the infusion of the uricosuric drugs sulfinpyrazone and benzbromarone. These results are consistent with competition for tubular reabsorption between MTX, ceftriaxone and sulfamethoxazole. The pharmacokinetic drug interactions observed occurred with therapeutic drug concentrations and thus may be clinically relevant.

The plasma concentrations and tissue distribution of thiopurines were studied in mice after oral administration of 50 mg/kg azathioprine (AZA) using HPLC analysis. Peak concentrations of AZA and three other thiopurine metabolites in plasma [thiouric acid (TUA) >6-mercaptopurine (6-MP)>AZA>8-hydroxy-AZA] were observed as early as 10 min after drug application, thus indicating fast absorption and extensive metabolism of AZA, and were followed by a rapid decline. The extraction of thiopurines from organs (intestinal mucosa, liver, kidney, testes, spleen, and bone marrow) and from red blood cells (RBCs) was preceded by an acid hydrolysis procedure resulting in the release of thiopurine bases from their corresponding ribonucleotides. 6-MP, 6-thioxanthine (6-TX), 6-thioguanine (6-TG), TUA, and 8-hydroxy-6-MP (8-OH-6-MP) were extracted from the organs, whereas only 6-MP and 8-OH-6-MP were found in the processed RBCs. Initially, high concentrations of TUA, the endpoint of metabolic AZA degradation, were detected in the intestinal mucosa and in the liver. This provides evidence for a first-pass metabolism of AZA in these two organs. The initial concentrations of 6-MP extracted from the organs were about 10-fold those found in plasma. This indicates rapid cellular uptake of 6-MP and an accumulation of 6-MP derivatives that can be explained by formation of the 6-MP ribonucleotide thioinosine monophosphate (TIMP). With the exception of plasma and RBCs, 6-TG, which may originate from intracellular 6-thioguanosine nucleotides (TGNs), was extracted from all organs examined in the study. From the sequence of appearance of 6-MP, 6-TX, and 6-TG extracted from spleen and bone marrow homogenates, it can be assumed that formation of TGN occurs via the nucleotide interconversion pathway TIMP ? 6-thioxanthosine monophosphate ? 6-thioguanosine monophosphate. The highest concentrations of 6-TG derivatives were found in the spleen and bone marrow. This correlates with the clinical and experimental observation that AZA cytotoxicity mainly affects bone-marrow stem cells and lymphocytes and supports the hypothesis (derived from in vitro experiments) that the incorporation of TGN into DNA is the cytotoxic mechanism of AZA and 6-MP.